And Nathan said to David, Thou art the man. Thus saith the LORD God of Israel, I anointed thee king over Israel, and I delivered thee out of the hand of Saul;

Wesley's Notes

12:7 Thus saith the Lord God - Nathan now speaks, not as a petitioner for a poor man, but as an ambassador from the great God.

Jamieson-Fausset-Brown Bible Commentary

2Sa 12:7-23. He Applies It to David, Who Confesses His Sin, and Is Pardoned.

7. Nathan said to David, Thou art the man-These awful words pierced his heart, aroused his conscience, and brought him to his knees. The sincerity and depth of his penitent sorrow are evinced by the Psalms he composed (Ps 32:1-11; 51:1-19; 103:1-22). He was pardoned, so far as related to the restoration of the divine favor. But as from his high character for piety, and his eminent rank in society, his deplorable fall was calculated to do great injury to the cause of religion, it was necessary that God should testify His abhorrence of sin by leaving even His own servant to reap the bitter temporal fruits. David was not himself doomed, according to his own view of what justice demanded (2Sa 12:5); but he had to suffer a quadruple expiation in the successive deaths of four sons, besides a lengthened train of other evils.

Matthew Henry's Concise Commentary

12:1-14 God will not suffer his people to lie still in sin. By this parable Nathan drew from David a sentence against himself. Great need there is of prudence in giving reproofs. In his application, he was faithful. He says in plain terms, Thou art the man. God shows how much he hates sin, even in his own people; and wherever he finds it, he will not let it go unpunished. David says not a word to excuse himself or make light of his sin, but freely owns it. When David said, I have sinned, and Nathan perceived that he was a true penitent, he assured him his sin was forgiven. Thou shalt not die: that is, not die eternally, nor be for ever put away from God, as thou wouldest have been, if thou hadst not put away the sin. Though thou shalt all thy days be chastened of the Lord, yet thou shalt not be condemned with the world. There is this great evil in the sins of those who profess religion and relation to God, that they furnish the enemies of God and religion with matter for reproach and blasphemy. And it appears from David's case, that even where pardon is obtained, the Lord will visit the transgression of his people with the rod, and their iniquity with stripes. For one momentary gratification of a vile lust, David had to endure many days and years of extreme distress.